January 2014 by Stuart Clark
Even the sun needs a break. A slowdown in solar activity has given us the first real clue about a period of dramatic solar behaviour 350 years ago.
In 2008, the sun entered a deep lull in magnetic activity. Spacecraft measurements show that this caused a belt of sluggish particles, known as the slow solar wind, to thicken. Produced near the sun's equator, the belt is normally narrow and ruched like a ballerina's tutu. The particles there flow at about 336 kilometres a second, as opposed to 550 kilometres a second in the fast solar wind produced closer to the sun's poles.
Because Earth orbits at a 14 degree tilt relative to the sun's equator, it passes in and out of these ruches during the year. As solar activity dwindles, the belt thickens, and we spend more time passing through it. The speed of the slow solar wind affects the temperature of Earth's upper atmosphere, and impacts climate.
To find out how severe such slowdowns might be, Mike Lockwood and Matt Owens at the University of Reading, UK, used the 2008 measurements to model the belt's thickness during the most extreme solar minimum on record: the Maunder Minimum. This lasted from 1645 to 1715 and corresponded to a minor ice age.
Not so windy
The model showed that the solar wind speed probably dropped to just 250 to 275 kilometres a second during this period. Earth spent six months of the 2008 minimum in the slow solar wind, but it probably spent all 70 years of the Maunder Minimum there.
Lockwood says the current weak cycle could herald the sun dipping intoanother extended period of inactivity. The temperature of Earth's upper atmosphere affects the speed at which satellite orbits decay, so a prolonged drop in the solar wind speed would cool and shrink the atmosphere and diminish the drag on satellites. This would exacerbate the space junkproblem because debris would be slower to re-enter Earth's atmosphere, where it burns up harmlessly.
But don't expect another mini ice age, says Lockwood. The Maunder Minimum was also characterised by an almost total lack of sunspots, which record how the solar dynamo, which creates the sun's magnetic field, is churning. We still do not understand that well enough to predict when another Maunder Minimum might occur.
"Until we know why the sun occasionally drops into these grand minima, we won't really understand the solar dynamo," says Lockwood.